Non-volatile semiconductor memory adapted to store a multi-valued data in a single memory cell

ABSTRACT

A non-volatile semiconductor memory device includes an electrically data rewritable non-volatile semiconductor memory cell and a write circuit for writing data in the memory cell, the write circuit writing a data in the memory cells by supplying a write voltage Vpgm and a write control voltage VBL to the memory cell, continuing the writing of the data in the memory cell by changing the value of the write control voltage VBL in response to an advent of a first write state of the memory cell and inhibiting any operation of writing a data to the memory cell by further changing the value of the write control voltage VBL to Vdd in response to an advent of a second write state of the memory cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.11/049,535 filed Feb. 3, 2005 now U.S. Pat. No. 7,088,616, which is aDivisional of U.S. application Ser. No. 10/656,139 filed on Sep. 8,2003, now U.S. Pat. No. 6,990,019, which is a divisional of 10/051,372filed on Jan. 22, 2002, now U.S. Pat. No. 6,643,188, and all are herebyincorporated by reference in their entirety. This application is basedupon and claims the benefit of priority from the prior Japanese PatentApplication No. 2001-397446, filed Dec. 27, 2001, the entire contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electrically data rewritable non-volatilesemiconductor memory device. More particularly, it relates to amulti-value flash memory adapted to store a multi-valued data in amemory cell.

2. Description of the Related Art

In a flash memory, the accumulated electric charge of the floating gateof a memory cell transistor is changed as the stored data is erased anda new data is written there. Then, as a result, the threshold value ischanged to store the data. For instance, the negative threshold valuemay be made to corresponds to a “1” data, whereas the positive thresholdvalue may be made to corresponds to a “0” data.

In recent years, multi-value flash memories adapted to store a pluralityof bits in a single memory cell have been developed to reduce the costper bit and/or increase the storage capacity. In a memory device adaptedto store two bits in a single memory cell, the memory cell has fourthreshold values depending on the data to be stored there.

A highly reliable memory device can be obtained by accuratelycontrolling the threshold values of each memory cell. “Fast and AccurateProgramming Method for Multi-level NAND EEPROMs, pp. 129-130, Digest of1995 Symposium on VLSI Technology” proposes a method of writing data,raising the write voltage Vpgm at a rate, in order to precisely controlthe threshold values of each memory cell.

With the method proposed in the above cited document, the width ofdistribution of each threshold value can be controlled theoretically toas small as 0.2V by raising the write voltage Vpgm at a rate of 0.2V/10μsec. Normally, the write voltage Vpgm is divided into a plurality ofwrite pulses and the voltage Vpgm of the pulses is raised stepwise at apredetermined rate. This technique provides an effect similar to that ofcontinuously raising the write voltage Vpgm. The threshold value ischecked after applying each pulse to the memory cell and the writeoperation is terminated when the threshold value has got to apredetermined verification level.

Meanwhile, micronization of processing dimensions is in progress. Thismeans that the gaps separating memory cells are made smaller and smallerto consequently give rise to various problems from the viewpoint ofmulti-valued flash memories. For instance, the distance separatingfloating gates is reduced to produce problems as pointed out below as aresult of micronization.

Imagine two memory cells A and B arranged side by side. Assume that thedata of the two memory cells are erased simultaneously and they are madeto have a threshold value of −3V. Then, firstly a data is written intothe memory cell A. As a result, its threshold value may be raised to0.5V to 1V. Subsequently, another data that is different from the datawritten into the memory cell A is written into the memory cell B. As thethreshold value of the memory cell B is raised to 1.5V to 2V, theelectric potential of the floating gate of the memory cell A falls andits threshold value is raised, say, to 1V to 1.5V as a result of thecapacitive coupling of the floating gates of the two memory cells.

In the above described instance, the difference of the threshold valuesof the memory cells A and B (read out margin) should be at least 0.5V.However, it is reduced to 0V as a result of the capacitive coupling ofthe floating gates of the two memory cells. Thus, the difference of thethreshold values necessary for discriminating two different data isreduced and the read out margin disappears.

How the threshold value of a memory cell storing a data written inadvance in a manner described above changes under the influence of awrite operation of another memory cell will be described below byreferring to FIGS. 1A through 1C of the accompanying drawing.

FIG. 1A shows the electric charge of the floating gate FG1 of a memorycell where the data stored there is erased and subsequently a new datais written. Electrons are accumulated in the floating gate FG1 of thememory cell where a data is written. In FIG. 1A, “— —” indicateselectrons. Subsequently, data are written in the memory cells locatedrespectively at the two sides of the first memory cell and havingrespective floating gates FG2, FG3. Then, a change occurs at thefloating gate FG1 of the first memory cell as shown in FIG. 1B. Theelectric potential of the memory cell where a data is written firstfalls and its threshold value rises as shown in FIG. 1C because of theelectrostatic capacitive coupling of the neighboring floating gates FG2,FG3. Then, as a result, the threshold value of the memory cell havingthe floating gate FG1 shows a wide distribution. In FIGS. 1A and 1B,reference symbol WL denotes the word line (control gate) arrangedcommonly for the memory cells having the floating gates FG1, FG2, FG3.

Thus, the technological development for reducing the distribution widthof the threshold value of a memory will become increasingly important inthe future in order to cope with this problem.

It may be conceivable to reduce the stepwise increment Dvpgm of thewrite voltage Vpgm in order to avoid this problem. For example, thedistribution width of the threshold value is reduced from 0.5V to 0.1Vto increase the write out margin by 0.4V by reducing the stepwiseincrement Dvpgm from 0.5V to 0.1V.

However, as the stepwise increment is reduced to ⅕ of the originalvalue, the number of pulses becomes five times as many as the originalnumber. Then, the write time will become five times as long as theoriginal value to give rise to a new problem.

Therefore, so far, any attempt at securing a write out margin andraising the reliability of a memory device is accompanied by the problemof an increased write time.

BRIEF SUMMARY OF THE INVENTION

In an aspect of the present invention, there is provided a non-volatilesemiconductor memory device comprises: an electrically data rewritablenon-volatile semiconductor memory cell; and a write circuit for writingdata in the memory cell, the write circuit writes a data in the memorycells by supplying a write voltage and a write control voltage to thememory cell, continues the writing of the data in the memory cell bychanging the supply of the write control voltage to the memory cell inresponse to an advent of a first write state of the memory cell andinhibits any operation of writing a data to the memory cell by furtherchanging the supply of the write control voltage to the memory cell inresponse to an advent of a second write state of the memory cell.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIGS. 1A through 1C are schematic illustrations of the sectional viewand distribution of a threshold value referred to for pointing out theproblem of the prior art;

FIG. 2 is a schematic block diagram of the first embodiment of flashmemory according to the invention, illustrating its overallconfiguration;

FIG. 3A is a schematic block diagram illustrating the internalconfiguration of the memory cell array in FIG. 2;

FIG. 3B is a circuit diagram of a NAND-type memory unit arranged in eachof the blocks of FIG. 3A;

FIG. 4 is a schematic cross sectional view of the memory cell array ofFIG. 2 taken along the column direction to show the structure of thedevice;

FIGS. 5A and 5B are schematic cross sectional views of the memory cellarray of FIG. 2 taken along the row direction to show the structure ofthe device;

FIG. 6 is a schematic block diagram of a principal part of the columncontrol circuit of FIG. 2, illustrating its configuration;

FIG. 7 is a graph illustrating the relationship between a multi-valueddata and the threshold value of a memory cell of the first embodiment ofmulti-value flash memory according to the invention;

FIG. 8 is a graph illustrating the changing threshold value of memorycells of a known flash memory and a data writing method adapted to usesuch a changing threshold value;

FIG. 9 is a graph illustrating the changing threshold value of a memorycell of the first embodiment of multi-value flash memory according tothe invention and a data writing method adapted to use such a changingthreshold value;

FIG. 10 is a graph illustrating the method for writing a higher orderpage data into a same memory cell and the change with time of thethreshold value of the memory of the first embodiment;

FIG. 11 is a graph illustrating the signal waveforms of different partsof the first embodiment of flash memory according to the invention whenwriting a lower order page data into a single memory cell;

FIG. 12 is a flow chart schematically illustrating the control algorithmof the first embodiment of flash memory according to the invention whenwriting a lower order page data into a single memory cell;

FIG. 13 is a flow chart schematically illustrating the control algorithmof the first embodiment of flash memory according to the invention whenwriting a higher order page data into a memory cell;

FIG. 14 is a flow chart schematically illustrating the control algorithmof the first embodiment of flash memory according to the invention forcontrolling the order of writing data into the blocks;

FIG. 15 is a flow chart schematically illustrating the control algorithmof the first embodiment of flash memory according to the invention whenreading the lower order page data stored in a memory cell;

FIG. 16 is a flow chart schematically illustrating the control algorithmof the first embodiment of flash memory according to the invention whenreading the higher order page data stored in a memory cell;

FIG. 17A is a graph illustrating the signal waveforms in a write step ofthe first embodiment of flash memory according to the invention;

FIG. 17B is a graph illustrating the signal waveforms in a write step ofthe second embodiment of flash memory according to the invention; and

FIG. 18 is a graph illustrating the signal waveforms of different partsof the third embodiment of flash memory according to the invention whenwriting a data into a single memory cell.

DETAILED DESCRIPTION OF THE INVENTION

Now, the present invention will be described in greater detail byreferring to the accompanying drawing that illustrates preferredembodiment of the invention.

FIG. 2 is a schematic block diagram of the first embodiment ofmulti-value flash memory according to the invention, illustrating itsoverall configuration;

Referring to FIG. 2, a plurality of flash memory cells, a plurality ofbit lines and a plurality of word lines are arranged in the memory cellarray 1. The flash memory cells are arranged in the form of a matrix.

A column control circuit 2 and a row control circuit 3 are arrangedadjacently relative to the memory cell array 1. The column controlcircuit 2 controls the bit lines in the memory cell array 1 for erasingdata from, writing data into and reading data from memory cells.

The row control circuit 3 is used for selecting a word line in thememory cell array 1 and supplying a voltage necessary for erasing,writing and reading data.

Additionally, a source line control circuit 4 for controlling sourcelines of the memory cell array 1 and a P-well control circuit 5 forcontrolling the p-type wells for forming the memory cell array 1 arealso arranged near the memory cell array 1.

Data input/output buffer 6 is connected to a host by way of an externalI/O line. The data input/output buffer 6 is adapted to receive data tobe written, outputs read out data and receive address data and commanddata. The data to be written received by the data input/output buffer 6are forwarded to the column control circuit 2. The data input/outputbuffer 6 receives the read out data from the column control circuit 2.

An external address data is sent to the column control circuit 2 and therow control circuit 3 by way of state machine 8 in order to selectmemory cells in the memory cell array 1.

A command data from the host is sent to command interface 7. The commandinterface 7 receives a control signal from the host and determines ifthe data input to the data input/output buffer 6 is a data to bewritten, a command data or an address data. If it is a command data, thecommand interface 7 forwards the command to the state machine 8 asreceived command signal.

The state machine 8 controls the overall operation of the flash memory.It receives a command from the host for controlling the operation ofreading data, writing data and erasing data and also controls the datainput/output operation. The state machine 8 arranged a write counter PCfor counting the number of data writing operations to each of the memorycells.

FIG. 3A is a schematic block diagram illustrating the internalconfiguration of the memory cell array 1 in FIG. 2. The memory cells ofthe memory cell array 1 are divided into a number of blocks BLOCK0through BLOCK1023. A block is the smallest unit for an erasingoperation. Each of the blocks BLOCKi (i=0 through 1023) includes a totalof 8,512 NAND type memory units as shown in FIG. 3B.

In this embodiment, each of the NAND type memory units contains fourmemory cells M that are connected in series and further to a bit lineBLe or BLo at an end thereof by way of a selection gate S1 commonlyconnected to selection gate lines SGD i and to a common source lineC-source at the opposite end thereof by way of a selection gate S2commonly connected to selection gate lines SGS i.

Each memory cell M has a control gate, a floating gate, a source and adrain. The control gates of the four memory cell M of each NAND typememory unit are commonly connected to the corresponding one of the wordlines WL0 i through WL3 i.

Data are independently written into and read out from the even-numberedbit lines BLe and the odd-numbered bit lines BLo as counted from 0. Dataare simultaneously written into or read out from 4,256 memory cellsconnected to the even-numbered bit lines BLe out of the 8,512 memorycells whose control gates are connected to a single word line WL.

When each memory cell stores a 1-bit data, the 4,256 bits data stored in4,256 memory cells constitute a unit of page. Thus, when a single memorycell stores a 2-bit data, the 4,256 memory cells store data of twopages. Data of other two pages are stored in the 4,256 memory cellsconnected to the odd-numbered bit lines BLo. Data are written into orread out from the memory cells of a same page simultaneously.

FIG. 4 is a schematic cross sectional view of the memory cell array 1 ofFIG. 2 taken along the column direction to show the structure of thedevice. Referring to FIG. 4, an n-type well 11 is formed on a p-typesubstrate 10 and a p-type well 12 is formed in the n-type well 11. Eachmemory cell M comprises a source and a drain formed in an n-typediffusion layer 13, a floating gate FG arranged in a channel regionbetween the source and the drain by way of a tunnel oxide film and acontrol gate CG arranged on the floating gate FG by way of an insulatingfilm and operating as word line WL.

Each of the selection gates S1, S2 includes a source and a drain formedin the n-type diffusion layer 13 and a selection gate line SG having atwo-layer structure. Both the word line WL and the selection gate lineSG are connected to the row control circuit 3 in FIG. 2 and controlledby the output signal from the row control circuit 3.

Each NAND type memory unit including four memory cells M and selectiongates S1, S2 is connected at an end thereof to the metal wiring layer M0of the first layer by way of a contact hole CB1. The metal wiring layerM0 is connected to the metal wiring layer M1 of the second layeroperating as bit line BL by way of a via hole V1. The bit line BL isconnected to the column control circuit 2 in FIG. 2.

The NAND type memory unit is connected at the other end thereof to themetal wiring layer M2 of the first layer operating as common source lineC-source by way of still another contact hole CB2. The common sourceline C-source is connected to the source line control circuit 4 in FIG.2.

An n-type diffusion layer 14 is formed on the surface of the n-type well11, while a p-type diffusion layer 15 is formed on the surface of thep-type well 12. Both of the n-type diffusion layer 14 and the p-typediffusion layer 15 are connected to the metal wiring layer M3 of thefirst layer operating as well line C-p-well by way of respective contactholes CB3, CB4. The well line C-p-well is connected to the P wellcontrol circuit 5 in FIG. 2.

FIGS. 5A and 5B are schematic cross sectional views of the memory cellarray 1 taken along the row direction to show the structure of thedevice. As shown in FIGS. 5A and 5B, each memory cell is isolated fromthe remaining memory cells by element isolations STI.

As shown in FIG. 5A, in each memory cell, a floating gate FG is laid ona channel region by way of a tunnel oxide film 16. A word line WL islaid on the floating gate FG by way of an insulating film 17 that is anONO film.

As shown in FIG. 5B, the selection gate line SG has a two-layerstructure. The upper layer selection gate line SG and the lower layerselection gate line SG are connected to an end of the memory cell array1 or a predetermined number of bit lines.

FIG. 6 is a schematic block diagram of a principal part of the columncontrol circuit 2 of FIG. 2, illustrating its configuration.

In the column control circuit 2, a data storage circuit 20 is arrangedfor every two bit lines including an even-numbered bit line BLe and anodd-numbered bit line BLo having a same column number. In the columncontrol circuit 2, a sense amplifier is also arranged for the datastorage circuit 20 in order to write data into and read data from memorycells.

Referring to FIG. 6, an n-channel MOS transistor Qn1 is connected forcolumn selection between the data storage circuit 20 and theeven-numbered bit line BLe, whereas another n-channel MOS transistor Qn2is connected for column selection between the data storage circuit 20and the odd-numbered bit line BLo.

Either of the even-numbered bit line BLe or the odd-numbered bit lineBLo connected to each data storage circuit 20 is selected and connectedto the data storage circuit 20 to control the operation of writing adata or that of reading a data. More specifically, when signal EVENBL isat level H and signal ODDBL is at level L, the MOS transistor Qn1 ismade electrically conductive to select the even-numbered bit line BLe,which bit line BLe is then connected to the data storage circuit 20.When, on the other hand, when signal EVENBL is at level L and signalODDBL is at level H, the MOS transistor Qn2 is made electricallyconductive to select the odd-numbered bit line BLo, which bit line BLois then connected to the data storage circuit 20. Note that the signalEVENBL is supplied to all the n-channel MOS transistors for columnselection connected to the even-numbered bit lines BLe, whereas thesignal ODDBL is supplied to all the n-channel MOS transistors for columnselection connected to the odd-numbered bit lines BLo. The unselectedbit lines BL are controlled by some other circuit (not shown).

Each data storage circuit 20 includes three binary data storage sectionsDS1, DS2, DS3, of which the data storage section DS1 is connected to thedata input/output buffer 6 by way of an internal data input/output line(I/O line) and stores an externally input data to be written or a readout data to be externally output, while the data storage section DS2stores the detection outcome of a write verify operation for confirmingthe threshold value of a memory cell after a write operation and thedata storage section DS3 temporarily stores the data of a memory cell atthe time of writing it and at the time of reading it.

FIG. 7 is a graph illustrating the relationship between a multi-valueddata and the threshold value of a memory cell of the first embodiment ofmulti-value flash memory according to the invention.

Now, the operation of the embodiment of multi-valued flash memoryaccording to the invention and having the above described configurationwill be described below by referring to FIG. 7. Assume that each memorycell of this embodiment is adapted to store two bits or a four-valueddata. It will be appreciated that a 2-bit data is “11”, “10”, “01” or“00”. The two bits belong respectively to different row addresses(different pages).

A four-valued data is stored in a memory cell with different thresholdvalues. Referring to FIG. 7, assume that a data showing the lowestthreshold value (e.g., the threshold voltage is negative) represents“11” and a data showing the second lowest threshold value (e.g., thethreshold voltage is positive) represents “10”, while a data showing thethird lowest threshold value (e.g., the threshold voltage is positive)represents “01” and a data showing the highest threshold value (e.g.,the threshold voltage is positive) represents “00”.

After an erasing operation, the data in the memory cell is “11”. If thedata of the lower order page written into this memory cell is “0”, thestate of the memory cell shifts from “11” to “10” as a result of thewriting operation. If the data written into this memory cell is “1”, thestate of the memory cell remains to be “11”.

Then, the data of the higher order page is written into the memory cell.If the written data is “1”, the state of the memory cell remain from“11” or “10”. If the written data is “0”, the state of the memory cellshift either from “11” to “01” or from “10” to “00”.

During a write operation, the data written into a memory cell is readout and a so-called write verify operation is conducted to verify if thewriting operation is satisfactory.

The data read out by the sense amplifier is regarded as “11” if thethreshold value is not higher than 0V and as “10 if the threshold valueis not lower than 0V and not higher than 1V, whereas the data isregarded as “01” if the threshold value is not lower than 1V and nothigher than 2V and as “00” if the threshold value is not lower than 2V.

Thus, four-value threshold values are used for storing a 2-bit data in amemory cell. In actual devices, the performance of the memory cells canvary from memory cell to memory cell and hence their threshold valuescan also vary. If they vary to a large extent, it will be no longerpossible to identify the data stored in each memory cell and a wrongdata may be read out.

This embodiment of multi-valued flash memory is adapted to suppressdispersion of threshold value in a manner as indicated by a solid linein FIG. 7 unlike the dispersion of threshold value observed in knownflash memories as indicated by broken lines in FIG. 7. This point willbe describe in detail to below.

Table 1 shows typical voltages of various parts of the first embodimentof multi-valued flash memory in erase, write, read and write verifyoperations. Note that, the values shown in Table 1 are obtained when theword line WL2 and the even-numbered bit lines BLe are selected for writeand read operations.

TABLE 1 First step Second step Write Erase write write inhibit “10” read“01” read “00” read BLe Floating 0 V 0.4 V   Vdd H or L H or L H or LBLo Floating Vdd Vdd Vdd   0 V   0 V   0 V SGD Floating Vdd Vdd Vdd 4.5V 4.5 V 4.5 V WL3 0 V 10 V  10 V  10 V  4.5 V 4.5 V 4.5 V WL2 0 V VpgmVpgm Vpgm   0 V   1 V   2 V WL1 0 V 0 V 0 V 0 V 4.5 V 4.5 V 4.5 V WL0 0V 10 V  10 V  10 V  4.5 V 4.5 V 4.5 V SGS Floating 0 V 0 V 0 V 4.5 V 4.5V 4.5 V C-source Floating 0 V 0 V 0 V   0 V   0 V   0 V C-p-well 20 V  0V 0 V 0 V   0 V   0 V   0 V “10” first “10” second “01” first “01”second “00” first “00” second step write step write step write stepwrite step write step write verify verify verify verify verify verifyBLe H or L H or L H or L H or L H or L H or L BLo   0 V   0 V   0 V   0V   0 V   0 V SGD 4.5 V 4.5 V 4.5 V 4.5 V 4.5 V 4.5 V WL3 4.5 V 4.5 V4.5 V 4.5 V 4.5 V 4.5 V WL2 0.2 V 0.4 V 1.2 V 1.4 V 2.2 V 2.4 V WL1 4.5V 4.5 V 4.5 V 4.5 V 4.5 V 4.5 V WL0 4.5 V 4.5 V 4.5 V 4.5 V 4.5 V 4.5 VSGS 4.5 V 4.5 V 4.5 V 4.5 V 4.5 V 4.5 V C-source   0 V   0 V   0 V   0 V  0 V   0 V C-p-well   0 V   0 V   0 V   0 V   0 V   0 V

For an erase operation, 20V and 0V are supplied respectively to thep-type well 12 (well line C-p-well) and all the word lines WL0 of theselected block. Electrons are discharged from the floating gates FG ofall the memory cells M of the block so that the threshold value becomesnegative to show a state of “11”. While the word lines and the bit linesBL of the unselected blocks are brought to an electrically floatingstate, they show a voltage level close to 20V as a result of thecapacitive coupling with the p-type well 12.

For writing a data, a first step operation, a second step operation anda write inhibiting operation are conducted sequentially. Firstly,program voltage (write voltage) Vpgm of about 14V to 20V is supplied tothe selected word line WL2. A high voltage such as 10V is supplied toeach of the unselected word lines, including, say, the word line WL3, ofthe memory cells located at the bit line side relative to the selectedmemory cells in order to make the memory cells connected to the wordline WL3 electrically conductive. On the other hand, a low voltage suchas 0V is supplied to each of the unselected word lines, including, saythe word line WL1, of the memory cells located at the side of the wellline C-p-well relative to the selected memory cells in order make thememory cells connected to the word line WL1 electrically non-conductive.The selected bit lines BLe is supplies a voltage such as 0V. As aresult, the 0V supplied to the selected bit lines BLe are transferred tothe drains of the selected memory cells and the electric potential ofthe floating gates FG is raised by the capacitive coupling of thecontrol gates CG and that of the floating gates FG so that electrons areinjected into the floating gates FG from the drain by way of the tunneloxide film (the tunnel oxide film 16 of FIG. 5A) due to the tunnelingphenomenon and the threshold value is rapidly raised (the first stepwrite operation). The voltage of the bit lines BLe is raised to 0.4V tosuppress the rate at which the threshold value rises in a writeoperation (the second step write operation). The bit lines BLe are madeto show a sufficiently high voltage, e.g., the supply voltage Vdd (up to3V) for completely blocking the rise of the threshold value (writeinhibition).

A read operation is conducted by sequentially supplying different readvoltages (0V, 1V, 2V) to the selected word line WL2. A voltage thatmakes the unselected memory cells electrically conductive, typically4.5V, is supplied to the unselected remaining word lines. If thethreshold value of the selected memory cells is lower than the readvoltage, the bit lines BLe and the common source line C-source are madeelectrically communicative with each other so that an electric currentflows through them to bring the electric potential of the bit lines BLeto a relatively low level, or level L. If, on the other hand, thethreshold value of the selected memory cells is higher the read voltage,the bit lines BLe and the common source line C-source are madeelectrically non-communicative with each other to bring the electricpotential of the bit lines BLe to a relatively high level, or level H.The read voltage is typically made equal to 0V and a read operation isconducted (to read “10”) for checking if the electric potential of amemory cell is higher than the threshold value corresponding to thestate of “10” or not. The read voltage is typically made equal to 1V anda read operation is conducted (to read “01”) for checking if theelectric potential of a memory cell is higher than the threshold valuecorresponding to the state of “01” or not. The read voltage is typicallymade equal to 2V and a read operation is conducted (to read “00”) forchecking if the electric potential of a memory cell is higher than thethreshold value corresponding to the state of “00” or not.

A data is written into a memory cell in the state of “10” so as to makethe threshold value not smaller than 0.4V in order to provide a readmargin of 0.4V for the read voltage of 0V. Thus, the operation ofwriting “10” is inhibited when the threshold value of the memory cellhas got to 0.4V as a result of a write verify operation.

Conventional devices comparable to this embodiment are only adapted tocheck if the threshold value has got to 0.4V or not so that thethreshold value shows a relatively broad distribution width as shown inFIG. 7.

To the contrary, this embodiment of the present invention is adapted tocheck if the threshold value has got to a level slightly lower than thetarget threshold value or not and the rate at which the threshold valuerises is suppressed in the second step write operation. Therefore, it isnow possible to narrow the distribution width of the threshold value asindicated by the solid line in FIG. 7. The above description alsoapplies to the states of “01” and “00”.

A write verify operation is conducted by sequentially supplyingdifferent verify voltages, e.g., 0.2V, 0.4V, 1.2V, 1.4V, 2.2V, 2.4V tothe selected word line WL2. If the threshold value of the selectedmemory cells is lower than the verify voltage, the bit lines BLe and thecommon source line C-source are made electrically communicative witheach other so that an electric current flows through them to bring theelectric potential of the bit lines BLe to a relatively low level, orlevel L. If, on the other hand, the threshold value of the selectedmemory cells is higher than the verify voltage, the bit lines BLe andthe common source line C-source are made electrically non-communicativewith each other to bring the electric potential of the bit lines BLe toa relatively high level, or level H.

If the target threshold value of the memory cell is 0.4V, the verifyvoltage is reduced typically to 0.2V for a write verify operation inorder to check if the threshold value of the memory cell is higher thana level slightly lower than the target threshold value, which is 0.2V inthis embodiment, or not (the first step operation of write verify “10”).The verify voltage is made equal to 0.4V and a write verify operation isconducted in order to check if the threshold value of the memory cell ishigher than 0.4 or not (the second step operation of write verify “10”).

If the target threshold value of the memory cell is 1.4V, the verifyvoltage is reduced typically to 1.2V for a write verify operation inorder to check if the threshold value of the memory cell is higher thana level slightly lower than the target threshold value, which is 1.2V inthis embodiment, or not (the first step operation of write verify “01”).The verify voltage is made equal to 1.4V and a write verify operation isconducted in order to check if the threshold value of the memory cell ishigher than 1.4V or not (the second step operation of write verify“01”).

If the target threshold value of the memory cell is 2.4V, the verifyvoltage is reduced to 2.2V for a write verify operation in order tocheck if the threshold value of the memory cell is higher than a levelslightly lower than the target threshold value, which is 2.2V in thisembodiment, or not (the first step operation of write verify “00”). Theverify voltage is made equal to 2.4V and a write verify operation isconducted in order to check if the threshold value of the memory cell ishigher than 2.4 or not (the second step operation of write verify “00”).

FIG. 8 is a graph illustrating the changing threshold value of memorycells of a known flash memory and a data writing method adapted to usesuch a changing threshold value. In FIG. 8, the small white squaresindicate the threshold value and the write control voltage (the voltageof the bit line BL) to be supplied to a memory cell where a data can beeasily written, whereas the small black squares indicate the thresholdvalue and the write control voltage (the voltage of the bit line BL) tobe supplied to a memory cell where a data can be hardly written. The twomemory cells stores the data of a same page. The data are erased fromboth of them in the initial state and they show a negative thresholdvalue.

The write voltage Vpgm is divided into a number of pulses and the pulsesare made to rise stepwise typically by 0.2V at a time. In other words,the write voltage Vpgm increased with a stepwise increment Dvpgm of 0.2Vper pulse.

As the voltage of the bit line BL that is the write control voltage ismade equal to 0V, the threshold value rises at a rate of 0.2V/pulsewhich is equal to the increment of the write voltage Vpgm after severalpulses. A write verify operation is conducted after the application ofeach write pulse and the write operation is inhibited at each memorycell whose threshold value, becomes to a bit line voltage Vdd of thememory cell detected to have got to the level of the write verifyvoltage. Thus, the threshold value shows a distribution width of 0.2V.

FIG. 9 is a graph illustrating the changing threshold value of a memorycell of the first embodiment of multi-value flash memory according tothe invention and a data writing method adapted to use such a changingthreshold value. As in the case of FIG. 8, the small white squaresindicate the threshold value and the write control voltage (the voltageof the bit line BL) to be supplied to a memory cell where a data can beeasily written, whereas the small black squares indicate thresholdvalues and a write control voltage (the voltage of the bit line BL) tobe supplied to a memory cell where a data can be hardly written. The twomemory cells stores the data of the respective columns of a same page.The data are erased from both of them in the initial state and they showa negative threshold value.

The write voltage Vpgm is divided into a number of pulses and the pulsesare made to rise stepwise typically by 0.2V at a time. In other words,the write voltage Vpgm increases with a stepwise increment Dvpgm of 0.2Vper pulse.

The voltage of the bit line BL that is the write control voltage is madeequal to 0V and a first step write operation is conducted. In the firststep write operation, the threshold value rises at a rate of 0.2V/pulsewhich is equal to the increment of the write voltage Vpgm after thesupplied several pulses. A first step write verify operation or a secondstep write verify operation is conducted after the application of eachwrite pulse.

The voltage of the bit line of the memory cell whose threshold value hasgot to the first step write verify voltage is subsequently increased to0.4V and the second step write operation is conducted on a memory cellby memory cell basis. The voltage of the bit line of the memory cellwhose threshold value has got to the second step write verify voltage issubsequently brought to Vdd to inhibit any write operation on a memorycell by memory cell basis.

In the second step write operation, the rising rate of the thresholdvalue is held lower than the 0.2V/pulse of the first step writeoperation for several pulses. In other words, while the voltage of thebit lines BL, or the write control voltage, is 0V in the first stepwrite operation, it rises to 0.4 in the second step write operation.Therefore, it is more difficult to write data in the second step writeoperation than in the first step write operation. The rising rate of thethreshold value in the second step write operation is typically heldwithin a range between 0V/pulse and 0.05V/pulse. In other words, thethreshold value shows a distribution width of as small as 0.05V in thesecond step write operation.

If the write pulse width is 20 μsec. and the time required for a writeverify operation is 5 μsec., the duration of a write operation isconventionally (20 μsec.+5 μsec.)×18 pulses=450 μsec.

Conventionally, the voltage increment Dvpgm of write voltage Vpgm needsto be made equal to 0.05V, or a quarter of 0.2V, in order to realize athreshold value distribution width of 0.05V. Then, the duration of awrite operation is 450 μsec×4=1800 μsec.

On the other hand, with this embodiment, as shown in FIG. 9, it ispossible to realize a threshold value distribution width of 0.05V byusing a voltage increment Dvpgm of 0.2V/pulse so that the duration of awrite time is (20 μsec.+5 μsec.+5 μsec.)×20 pulses=600 μsec.

Thus, the duration of the write operation necessary for realizing athreshold value distribution width of 0.05V in this embodiment isreduced to a third of that of the above known device.

“10” is written by using a “10” first step write verify voltage and a“10” second step write verify voltage respective for first step writeverify voltage and for the second step write verify voltage.

FIG. 10 is a graph illustrating the method for writing a higher orderpage data into a same memory cell and the change with time of thethreshold value of the memory of the first embodiment. As in the case ofFIGS. 8 and 9, the small white squares indicate the threshold value andthe write control voltage (the voltage of the bit line BL) to besupplied to a memory cell where a data can be easily written, whereasthe small black squares indicate threshold values and a write controlvoltage (the voltage of the bit line BL) to be supplied to a memory cellwhere a data can be hardly written. The two memory cells stores the dataof the respective columns of a same page.

The data in the memory cell whose write control voltage is indicated bywhite squares, where a data can be easily written, is erased in theinitial state and the memory cell shows a negative threshold value.Assume that a date is written in the memory cell to make it show to showa “01” state. A data is already written in the memory cell whose writecontrol voltage is indicated by black squares to make it show a “10”state in the initial state. Assume that a data is written to the memorycell to make it show a “00” state.

The write voltage Vpgm is divided into a number of pulses and the pulsesare made to rise stepwise typically by 0.2V at a time. In other words,the write voltage Vpgm increases with a stepwise increment Dvpgm of 0.2Vper pulse.

The voltage of the bit line BL that is the write control voltage is madeequal to 0V and a first step write operation is conducted. In the firststep write operation, the threshold value rises at a rate of 0.2V/pulsewhich is equal to the increment of the write voltage Vpgm after severalpulses. A “01” first step write verify operation is conducted after theapplication of each write pulse. After the write operation using athreshold value slightly lower than the target threshold value, a “01”second step write verify operation is conducted after the application ofeach write pulse. Thereafter, a “00” first step write verify operationand a “00” second step write verify operation are conducted.

When the threshold value of the memory cell indicated by white squaresis detected to have got to the “01” first step write verify voltage,subsequently the bit line voltage is made equal to 0.4V and the processproceeds to the second step write operation. When the threshold value ofthe memory cell indicated by black squared is detected to have got tothe “00” first step write verify voltage, subsequently the bit linevoltage is made equal to 0.4V and the process proceeds to the secondstep write operation.

Furthermore, when the threshold value of the memory cell indicated bywhite squares is detected to have got to the “01” second step writeverify voltage, subsequently the bit line voltage is made equal to Vddand the write operation is inhibited. Finally, when the threshold valueof the memory cell indicated by black squares is detected to have got tothe “00” second step write verify voltage, subsequently the bit linevoltage is made equal to Vdd and the write operation is inhibited.

After the second step write operation starts for both the data “01” andthe data “00”, the increment of the threshold value is typically heldwithin a range between about 0V/pulse and 0.05V/pulse for several pulsesof the write voltage. Therefore, the threshold value shows only adistribution width of 0.05V.

FIG. 11 is a graph illustrating the signal waveforms of different partsof the first embodiment of flash memory according to the invention whenwriting a lower order page data into a single memory cell.

Referring to FIG. 11, the write step continues from time tp0 to timetp7. A write pulse is applied during this period. The “10” first stepwrite verify operation continues from time tfv0 to time tfv6. Then, theperiod of time from time tsv0 to time tsv6 is assigned to the “10”second step write verify operation. In this instance, it is assumed thatthe word line WL2 and the even-numbered bit lines BLe are selected.

In the write step, the voltage of the bit lines BLe that is the writecontrol voltage is brought to 0V for the first step write operation andto 0.4V for the second step write operation, whereas it is brought toVdd (e.g., 2.5V) when any write operation is inhibited.

In each write verify period, firstly the bit lines BLe is chargedtypically to 0.7V. Thereafter, when the selected word line WL2 has getsto the write verify voltage, the bit lines BLe is held to 0.7V if thethreshold value of the memory cell has got to the write verify voltagebut the voltage of the bit lines BLe is reduced toward 0V if thethreshold value of the memory cell has not got to the write verifyvoltage. If the threshold value of the memory cell has got to the writeverify voltage or not can be detected by observing the voltage of thebit lines BLe by means of a sense amplifier at timing of time tfv4 ortsv4. If the threshold value of the memory cell has got to the writeverify voltage, the detecting operation is successfully completed.

FIG. 12 is a flow chart schematically illustrating the control algorithmof the first embodiment of flash memory according to the invention whenwriting a lower order page data into a single memory cell.

The control operation starts with receiving a data input command fromthe host and placing the data input command in the state machine 8 (S1).Then, the operation proceeds to receiving an address data from the hostand placing the address in the state machine 8 to select the page to beused for a write operation (S2). Thereafter, the operation proceeds to astep of receiving data to be written in a page and storing themcorrespondingly in the respective data storage sections DS1 (S3).Subsequently, the operation proceeds to a step of receiving a writecommand issued from the host and placing the write command in the statemachine 8 (S4). As the write command is placed, the operation of StepsS5 through S16 is automatically started by the state machine 8 in theinside.

The data stored in the data storage sections DS1 are copied respectivelyto the corresponding data storage sections DS2 (S5). Thereafter, 12V isselected for the initial value of the write voltage Vpgm and the writecounter PC is set to 0 (S6). If the data in the data storage sectionsDS1 are “0”s and the data in the data storage sections DS2 are also“0”s, they indicate a first step write operation and, therefore, thevoltage of the bit lines BLe that is the write control voltage isreduced to 0V. If, on the other hand, the data in the data storagesections DS1 are “0”s and the data in the data storage sections DS2 are“1”s, they indicate a second step write operation and, therefore, thevoltage of the bit lines BLe that is the write control voltage isbrought to 0.4V. If, finally, the data in the data storage sections DS1are “1”s and the data in the data storage sections DS2 are also “1”s,they indicate write inhibition and, therefore, the voltage of the bitlines BLe that is the write control voltage is brought to Vdd (S7).

Then, the operation proceeds to the write step of applying a write pulseto the memory cells for storing the data of a page by using the selectedwrite voltage Vpgm and the write control voltage (S8). In the next step,if all the data stored in the data storage sections DS2 are “1”s or notis checked and, if they are all “1”s, it is determined that the statusof the first step is satisfactory whereas, if all the data stored in thedata storage sections DS2 are not “1”s, it is determined that the statusof the first step is not satisfactory (S9). As will be describedhereinafter, if all the data stored in the data storage sections DS2 are“1”s, there is no memory cell where the first step write operation isconducted in the preceding write step (S8).

If the status of the first step is satisfactory, a “10” first step writeverify operation is started (S10) and the data of the data storagesections DS2 corresponding to only the memory cells where the detectionoutcome is satisfactory out of the memory cells for storing the data ofa page are shifted from is “0”s to “1”s. The data storage sections DS2storing “1”s are made to keep on storing “1”s.

When the status of the first step is satisfactory or when the “10” firststep write verify operation is completed, a “10” second step writeverify operation is started (S11). The data of the data storage sectionsDS1 corresponding to only the memory cells where the detection outcomeis satisfactory out of the memory cells for storing the data of a pageare shifted from “0”s to “1”s. The data storage sections DS1 storing“1”s are made to keep on storing “1”s.

After the “10” second step write verify operation, if all the datastored in the data storage sections DS1 are “1”s or not is checked and,if they are all “1”s, it is determined that the status of the secondstep is satisfactory whereas, if all the data stored in the data storagesections DS2 are not “1”s, it is determined that the status of thesecond step is not satisfactory (S12). If the status of the second stepis satisfactory, it is judged that the write operation has completedsuccessfully and the status of the write operation is rated assatisfactory to terminate the write operation (S13).

If, on the other hand, the status of the second step is notsatisfactory, the write counter PC is checked (S14). If the reading ofthe write counter PC is not less than 20, it is judged that the statusof the write operation is failure and the write operation is terminatedunsuccessfully (S15). If the reading of the write counter PC is notgreater than 20, the reading of the write counter PC is incremented byone and the write voltage Vpgm is raised by 0.2V (S16). Then, theoperation is moved back to Step S7 and then the write operation of StepS8 is retried. It will be appreciated that the norm for the writeoperation is not necessarily be 20 and some other norm may be selectedif appropriate.

Table 2 shows the relationship between the data of the data storagesections DS1 and DS2 before and after the “10” first step write verifyoperation and the threshold value (Vt) of the corresponding memory cellsof the write algorithm illustrated in FIG. 12.

TABLE 2 DS1/DS2 data DS1/DS2 after n-th “10” first step write verifyMemory cell threshold value Vt When lower When higher than 0.2 V than0.2 V DS1/DS2 data DS1/DS2 0/0 0/0 0/1 before n-th “10” first 0/1 0/10/1 step write verify 1/1 1/1 1/1

Immediately before the n-th “10” first step write verify operation, thedata of the data storage sections DS1 and DS2 are one of thecombinations of 0/0, 0/1 and 1/1. The combination of 0/0 indicates thatthe threshold value of the memory cells has not got to the “10” firststep write verify voltage by the n−1-th write step. The combination of0/1 indicates that the threshold value of the memory cells has got tothe “10” first step write verify voltage but not to the “10” second stepwrite verify voltage by the n−1-th write step. The combination of 1/1indicates that the threshold value of the memory cells has got to the“10” second step write verify voltage by the n−1-th write step. It isnot possible that the threshold value of the memory cells has got to the“10” second step write verify voltage but not to the “10” first stepwrite verify voltage by the n−1-th write step so that the combination of1/0 does not exists in this embodiment.

Immediately before the first “10” first step write verify operation, thedata of the data storage sections DS1 and DS2 are either of thecombinations of 0/0 and 1/1.

If the threshold value of the memory cells has not got to 0.2V which isthe “10” first step write verify voltage by the n-th write step, thedetection outcome of the “10” first step write verify operation is notsatisfactory so that the data in the data storage sections DS2 are notchanged. If, on the other hand, the threshold value of the memory cellshas got to 0.2V, the detection outcome of the “10” first step writeverify operation is satisfactory so that the data in the data storagesections DS2 are shifted to “1”s. The data storage sections DS2 storing“1”s are made to keep on storing “1”s.

Table 3 shows the relationship between the data of the data storagesections DS1 and DS2 before and after the “10” second step write verifyoperation and the threshold value of the corresponding memory cells ofthe write algorithm illustrated in FIG. 12.

TABLE 3 DS1/DS2 data DS1/DS2 after n-th “10” second step write verifyMemory cell threshold value Vt When lower When higher than 0.4 V than0.4 V DS1/DS2 data DS1/DS2 0/0 0/0 — before n-th “10” second 0/1 0/1 1/1step write verify 1/1 1/1 1/1

Immediately before the n-th “10” second step write verify operation, thedata of the data storage sections DS1 and DS2 are one of thecombinations of 0/0, 0/1 and 1/1. The combination of 0/0 indicates thatthe threshold value of the memory cells has not got to the “10” firststep write verify voltage after the end of the n-th write step. Thecombination of 0/1 indicates that the threshold value of the memorycells has got to the “10” first step write verify voltage by the n-thwrite step but not to the “10” second step write verify voltage by then−1th write step. The combination of 1/1 indicates that the thresholdvalue of the memory cells has got to the “10” second step write verifyvoltage by the end of the n−1-th write step.

It is not possible that the threshold value of the memory cells has gotto the “10” second step write verify voltage by the n−1th write step butnot to the “10” first step write verify voltage by the n-th write stepso that the combination of 1/0 does not exists in this embodiment.

If the threshold value of the memory cells has not got to 0.4V which isthe “10” second step write verify voltage by the n-th write step, thedetection outcome of the “10” second step write verify operation is notsatisfactory so that the data in the data storage sections DS1 are notchanged. If, on the other hand, the threshold value of the memory cellshas got to 0.4V, the detection outcome of the “10” second step writeverify operation is satisfactory so that the data in the data storagesections DS1 are shifted to “1”s. The data storage sections DS2 storing“1”s are made to keep on storing “1”s. The combination of 0/0 will notbe changed by the “10” second write verify operation.

FIG. 13 is a flow chart schematically illustrating the control algorithmof the first embodiment of flash memory according to the invention whenwriting a higher order page data into a memory cell.

Referring to FIG. 13, the control operation starts with receiving a datainput command from the host and placing the data input command in thestate machine 8 (S1). Then, the operation proceeds to receiving anaddress data from the host and placing the address in the state machine8 to select the page to be used for a write operation (S2). Thereafter,the operation proceeds to a step of receiving data to be written in apage and storing them correspondingly in the respective data storagesections DS1 (S3). Subsequently, the operation proceeds to a step ofreceiving a write command issued from the host and placing the writecommand in the state machine 8 (S4). As the write command is placed, theoperation of Steps S5 through S20 is automatically started by the statemachine 8 in the inside.

Firstly, a “10” write operation is started (S5) and the operation issatisfactory (the data of the memory cells are “10”s, “0”s are stored inthe corresponding data storage sections DS3. If the operation is notsatisfactory, “1” are stored in the corresponding data storage sectionsDS3. Thereafter, the data stored in the data storage sections DS1 arecopied respectively to the corresponding storage sections DS2 (S6).Then, 14V is selected for the initial value of the write voltage Vpgmand the write counter PC is set to 0 (S7). If the data in the datastorage sections DS1 are “0”s and the data in the data storage sectionsDS2 are also “0”s, they indicate a first step write operation and,therefore, the voltage of the bit lines BL that is the write controlvoltage is set to 0V. If, on the other hand, the data in the datastorage sections DS1 are “0”s and the data in the data storage sectionsDS2 are “1”s, they indicate a second step write operation and,therefore, the voltage of the bit lines BL that is the write controlvoltage is set to 0.4V. If, finally, the data in the data storagesections DS1 are “1”s and the data in the data storage sections DS2 arealso “1”s, they indicate write inhibition and, therefore, the voltage ofthe bit lines BL that is the write control voltage is set to Vdd (S8).Then, the operation proceeds to the write step of applying a write pulseto the memory cells for storing the data of a page by using the selectedwrite voltage Vpgm and the write control voltage (S9).

In the next step, in all the data storage circuits 20 where “0”s arestored in the data storage sections DS3, it is checked if all the datastored in the data storage sections DS2 are “1”s or not and, if they areall “1”s, it is determined that the status of the “00” first step issatisfactory whereas, if all the data stored in the data storagesections DS2 are not “1”s, it is determined that the status of the “00”first step is not satisfactory (S10). If all the data stored in the datastorage sections DS2 are “1”s, there is no memory cell where the “00”first step write operation is conducted in the preceding write step(S9).

If the status of the “00” first step is not satisfactory, a “00” firststep write verify operation is started (S11) and the data of the datastorage sections DS2 corresponding to only the memory cells where thedetection outcome is satisfactory out of the memory cells for storingthe data of a page are shifted from “0”s to “1”s, provided that the datain the data storage sections DS3 are “0”. The data storage sections DS2storing “1”s are made to keep on storing “1”s.

When the status of the “00” first step is satisfactory or when the “00”first step write verify operation is completed, a “00” second step writeverify operation is started (S12). The data of the data storage sectionsDS1 corresponding to only the memory cells where the detection outcomeis satisfactory out of the memory cells for storing the data of a pageare shifted from “0”s to “1”s, provided that the data in the datastorage section DS3 are “0”s. The data storage sections DS1 storing “1”sare made to keep on storing “1”s.

Thereafter, in all the data storage circuits 20 where “0”s are stored inthe data storage sections DS3, it is checked if all the data stored inthe data storage sections DS2 are “1”s or not is checked, if they areall “1”s, it is determined that the status of the “01” first step issatisfactory whereas, if all the data stored in the data storagesections DS2 are not “1”s, it is determined that the status of that stepis not satisfactory (S13). As will be described hereinafter, if all thedata stored in the data storage sections DS2 are “1”s, there is nomemory cell where the first step write operation is conducted in thepreceding write step (S9).

If the status of the “01” first step is not satisfactory, a “01” firststep write verify operation is started (S14) and, in all the datastorage circuits 20 where “0”s are stored in the data storage sectionsDS3, the data of the data storage sections DS2 corresponding to only thememory cells where the detection outcome is satisfactory out of thememory cells for storing the data of a page are shifted from “0”s to“1”s, provided that the data in the data storage sections DS3 are “0”.The data storage sections DS2 storing “1”s are made to keep on storing“1”s.

When the status of the “01” first step is satisfactory or when the “10”first step write verify operation is completed, a “10” second step writeverify operation is started (S15). Then, in all the data storagecircuits 20 where “0”s are stored in the data storage sections DS3, thedata of the data storage sections DS1 corresponding to only the memorycells where the detection outcome is satisfactory out of the memorycells for storing the data of a page are shifted from “0”s to “1”s. Thedata storage sections DS1 storing “1”s are made to keep on storing “1”s.

After the “01” second step write verify operation, if all the datastored in the data storage sections DS1 are “1”s or not is checked and,if they are all “1”s, it is determined that the status of the secondstep is satisfactory whereas, if all the data are not “1”s, it isdetermined that the status of the second step is not satisfactory (S16).If the status of the second step is satisfactory, it is judged that thewrite operation has completed successfully and the status of the writeoperation is rated as satisfactory to terminate the write operation(S17). If, on the other hand, the status of the second step is notsatisfactory, the write counter PC is checked (S18). If the reading ofthe write counter PC is not less than 20, it is judged that the statusof the write operation is failure and the write operation is terminatedunsuccessfully (S19). If the reading of the write counter PC is notgreater than 20, the reading of the write counter PC is incremented byone and the write voltage Vpgm is raised by 0.2V (S20). Then, theoperation is moved back to Step S8 and then the write operation of StepS9 is retried. It will be appreciated that the norm for the writeoperation is not necessarily be 20 and some other norm may be selectedif appropriate.

Table 4 shows the relationship between the data of the data storagesections DS1, DS2 and DS3 before and after the “10” first step writeverify operation and the threshold value (Vt) of the correspondingmemory cells of the write algorithm illustrated in FIG. 13.

TABLE 4 DS1/DS2/DS3 data DS1/DS2/DS3 after n-th “01” first step writeverify Memory cell threshold value Vt When lower When higher than 1.2 Vthan 1.2 V DS1/DS2/DS3 data 0/0/1 0/0/1 0/1/1 DS1/DS2/D3 before 0/1/10/1/1 0/1/1 n-th “01” first 1/1/1 1/1/1 1/1/1 step write verify 0/0/00/0/0 0/0/0 0/1/0 0/1/0 0/1/0 1/1/0 1/1/0 1/1/0

Immediately before the n-th “01” first step write verify operation, thedata of the data storage sections DS1, DS2 and DS3 are one of thecombinations of 0/0/1, 0/1/1, 1/1/1, 0/0/0, 0/1/0 and 1/1/0. Thecombination of 0/0/1 indicates that the threshold value of the memorycells has not got to the “01” first step write verify voltage by then−1-th write step. The combination of 0/1/1 indicates that the thresholdvalue of the memory cells has got to the “01” first step write verifyvoltage but not to the “01” second step write verify voltage by then−1-th write step. The combination of 1/1/1 indicates that the thresholdvalue of the memory cells has got to the “01” second step write verifyvoltage by the n−1-th write step. It is not possible that the thresholdvalue of the memory cells has got to the “01” second step write verifyvoltage but not to the “01” first step write verify voltage by then−1-th write step so that the combination of 1/0/0 does not exists inthis embodiment.

If the threshold value of the memory cells has not got to 1.2V which isthe “01” first step write verify voltage by the n-th write step, thedetection outcome of the “01” second step write verify operation is notsatisfactory so that the data in the data storage sections DS2 are notchanged. If, on the other hand, the threshold value of the memory cellshas got to 1.2V, the detection outcome of the “01” first step writeverify operation is satisfactory so that the data in the data storagesections DS2 are shifted to “1”s. The data storage sections DS2 storing“1”s are made to keep on storing “1”s. The combinations of 0/0/0, 0/1/0and 1/1/0 do not constitute any objects of the first step write verifyoperation so that they are not changed.

Table 5 shows the relationship between the data of the data storagesections DS1, DS2 and DS3 before and after the “01” second step writeverify operation and the threshold value (Vt) of the correspondingmemory cells of the write algorithm illustrated in FIG. 13.

TABLE 5 DS1/DS2/DS3 data DS1/DS2/DS3 after n-th “01” second step writeverify Memory cell threshold value Vt When lower When higher than 1.4 Vthan 1.4 V DS1/DS2/DS3 data 0/0/1 0/0/1 — DS1/DS2/D3 before 0/1/1 0/1/11/1/1 n-th “01” second 1/1/1 1/1/1 1/1/1 step write verify 0/0/0 0/0/00/0/0 0/1/0 0/1/0 0/1/0 1/1/0 1/1/0 1/1/0

Immediately before the n-th “01” second step write verify operation, thedata of the data storage sections DS1, DS2 and DS3 are one of thecombinations of 0/0/1, 0/1/1, 1/1/1, 0/0/0, 0/1/0 and 1/1/0. Thecombination of 0/0/1 indicates that the threshold value of the memorycells has not got to the “01” first step write verify voltage after then-th write step. The combination of 0/1/1 indicates that the thresholdvalue of the memory cells has got to the “01” first step write verifyvoltage by the n-th write step but not to the “01” second step writeverify voltage by the n−1-th write step. The combination of 1/1/1indicates that the threshold value of the memory cells has got to the“01” second step write verify voltage by the n−1-th write step. It isnot possible that the threshold value of the memory cells has got to the“01” second step write verify voltage by the n−1-th write step but notto the “01” first step write verify voltage by the n-th write step sothat the combination of 1/0/1 does not exists in this embodiment.

If the threshold value of the memory cells has not got to 1.4V which isthe “01” second step write verify voltage by the n-th write step, thedetection outcome of the “01” second step write verify operation is notsatisfactory so that the data in the data storage sections DS1 are notchanged. If, on the other hand, the threshold value of the memory cellshas got to 1.4V, the detection outcome of the “01” second step writeverify operation is satisfactory so that the data in the data storagesections DS1 are shifted to “1”s. The data storage sections DS2 storing“1”s are made to keep on storing “1”s. The combination of 0/0/1 will notbe changed by the “01” second write verify operation. The combinationsof 0/0/0, 0/1/0 and 1/1/0 do not constitute any objects of the firststep write verify operation so that they are not changed.

Table 6 shows the relationship between the data of the data storagesections DS1, DS2 and DS3 before and after the “00” first step writeverify operation and the threshold value (Vt) of the correspondingmemory cells of the write algorithm illustrated in FIG. 13.

TABLE 6 DS1/DS2/DS3 data DS1/DS2/DS3 after n-th “00” first step writeverify Memory cell threshold value Vt When lower When higher than 2.2 Vthan 2.2 V DS1/DS2/DS3 data 0/0/1 0/0/1 — DS1/DS2/D3 before 0/1/1 0/1/1— n-th “00” first 1/1/1 1/1/1 — step write verify 0/0/0 0/0/0 0/1/00/1/0 0/1/0 0/1/0 1/1/0 1/1/0 1/1/0

Immediately before the n-th “00 first step write verify operation, thedata of the data storage sections DS1, DS2 and DS3 are one of thecombinations of 0/0/1, 0/1/1, 1/1/1, 0/0/0, 0/1/0 and 1/1/0. Thecombination of 0/0/0 indicates that the threshold value of the memorycells has not got to the “00” first step write verify voltage by then−1-th write step. The combination of 0/1/0 indicates that the thresholdvalue of the memory cells has got to the “00” first step write verifyvoltage but not to the “00” second step write verify voltage by then−1-th write step. The combination of 1/1/0 indicates that the thresholdvalue of the memory cells has got to the “00” second step write verifyvoltage. It is not possible that the threshold value of the memory cellshas got to the “00” second step write verify voltage but not to the “00”first step write verify voltage by the n−1-th write step so that thecombination of 1/0/0 does not exists in this embodiment.

If the threshold value of the memory cells has not got to 2.2V which isthe “00” first step write verify voltage by the n-th write step, thedetection outcome of the “00” first step write verify operation is notsatisfactory so that the data in the data storage sections DS2 are notchanged. If, on the other hand, the threshold value of the memory cellshas got to 2.2V by the n-th writer step, the detection outcome of the“00” first step write verify operation is satisfactory so that the datain the data storage sections DS2 are shifted to “1”s. The data storagesections DS2 storing “1”s are made to keep on storing “1”s. Thecombinations of 0/0/1, 0/1/1 and 1/1/1 do not constitute any objects ofthe first step; very operation so that they are not changed.

Table 7 shows the relationship between the data of the data storagesections DS1, DS2 and DS3 before and after the “00” second step writeverify operation and the threshold value (Vt) of the correspondingmemory cells of the write algorithm illustrated in FIG. 13.

TABLE 7 DS1/DS2/DS3 data DS1/DS2/DS3 after n-th “00” second step writeverify Memory cell threshold value Vt When lower When higher than 2.4 Vthan 2.4 V DS1/DS2/DS3 data 0/0/1 0/0/1 — DS1/DS2/D3 before 0/1/1 0/1/1— n-th “00” second 1/1/1 1/1/1 — step write verify 0/0/0 0/0/0 — 0/1/00/1/0 0/1/0 1/1/0 1/1/0 1/1/0

Immediately before the n-th “00” second step write verify operation, thedata of the data storage sections DS1, DS2 and DS3 are one of thecombinations of 0/0/1, 0/1/1, 1/1/1, 0/0/0, 0/1/0 and 1/1/0. Thecombination of 0/0/0 indicates that the threshold value of the memorycells has not got to the “00” first step write verify voltage after then-th write step. The combination of 0/1/0 indicates that the thresholdvalue of the memory cells has got to the “00” first step write verifyvoltage by the n-th write step but not to the “00” second step writeverify voltage by the n−1-th write step. The combination of 1/1/0indicates that the threshold value of the memory cells has got to the“00” second step write verify voltage by the n−1-th write step. It isnot possible that the threshold value of the memory cells has got to the“00 second step write verify voltage by the n−1-th write step but not tothe “00 first step write verify voltage by the n-th write step so thatthe combination of 1/0/0 does not exists in this embodiment.

If the threshold value of the memory cells has not got to 2.4V which isthe “00” second step write verify voltage by the n-th write step, thedetection outcome of the “00” second step write verify operation is notsatisfactory so that the data in the data storage sections DS1 are notchanged. If, on the other hand, the threshold value of the memory cellshas got to 2.4V, the detection outcome of the “00” second step writeverify operation is satisfactory so that the data in the data storagesections DS1 are shifted to “1”s. The data storage sections DS1 storing“1”s are made to keep on storing “1”s. The combination of 0/0/0 will notbe changed by the “00” second write verify operation. The combinationsof 0/0/1, 0/1/1 and 1/1/1 do not constitute any objects of the firststep; very operation so that they are not changed.

FIG. 14 is a flow chart schematically illustrating the control algorithmof the first embodiment of flash memory according to the invention forcontrolling the order of writing data into the blocks.

Firstly, the word line WL0 is selected and lower order data are writteninto a page constituted by a plurality of memory cells connected toeven-numbered bit lines. Secondly, lower order data are written into apage constituted by a plurality of memory cells connected toodd-numbered bit lines. Thirdly, higher order data are written into apage constituted by a plurality of memory cells connected toeven-numbered bit lines. Finally, higher order data are written into apage constituted by a plurality of memory cells connected toodd-numbered bit lines. Then, data are written in a similar manner bysequentially using the remaining word lines WL1, WL2, WL3, . . . ,observing the above sequence.

With this arrangement, the interference of the floating gates ofadjacent memory cells can be minimized. In other words, if a memory cellwhere a data is written subsequently shifts its state from “11” to “10”,from “11” to “01” or from “10” to “00”, a shift from “11” to “00” nevertakes place. The shift from “11” to “00” raises the threshold value ofadjacent memory cells most.

FIG. 15 is a flow chart schematically illustrating the control algorithmof the first embodiment of flash memory according to the invention whenreading the lower order page data stored in a memory cell.

The control operation starts with receiving a read command from the hostand placing the read command in the state machine 8 (S1). Then, theoperation proceeds to receiving an address data from the host andplacing the address in the state machine 8 to select the page to be usedfor a read operation (S2). As a result of the addressing, the operationof Steps S3 through S5 is automatically started by the state machine 8in the inside.

Firstly, a “01” read operation is started (S3). A voltage of 1V issupplied to the word line WL for the “01” read operation. “1” isproduced by the reading operation of the sense amplifier if thethreshold value of the memory cell is lower than the “01” data, whereas“0” is produced if the threshold value of the memory cell is higher than“01” data. The outcome of the read operation is stored in thecorresponding data storage section DS3. Thereafter, a “10” readoperation is started (S4). A voltage of 0V is supplied to the word lineWL for the “10” read operation. “1” is produced by the reading operationof the sense amplifier if the threshold value of the memory cell islower than the “10” data, whereas “0” is produced if the threshold valueof the memory cell is higher than “10” data. The outcome of the readoperation is stored in the corresponding data storage section DS2.Lastly, a “00” read operation is started (S5). A voltage of 2V issupplied to the word line WL for the “00” read operation. “1” isproduced by the reading operation of the sense amplifier if thethreshold value of the memory cell is lower than the “00” data, whereas“0” is produced if the threshold value of the memory cell is higher than“00” data. The lower order page data is produced by a logical operationusing the outcome of the “00” read operation and the data stored in thecorresponding data storage sections DS2 and DS3 and stored in thecorresponding data storage section DS1. The data stored in the datastorage section DS1 is output as lower order page data.

For example, if the outcome of the operation of reading “01” stored inthe data storage section DS3 is “1” and that of the operation of reading“10” stored in the data storage section DS2 is also “1”, “1” is producedby the logical operation using the lower order page data. If the outcomeof the operation of reading “01” stored in the data storage section DS3is “1” and that of the operation of reading “10” stored in the datastorage section DS2 is “0”, “0” is produced by the logical operationusing the lower order page data. If the outcome of the operation ofreading “01” stored in the data storage section DS3 is “0” and that ofthe operation of reading “00” is also “0”, “0” is produced by thelogical operation using the lower order page data. The outcome of theoperation of reading “01” stored in the data storage section DS3 is “0”and that of the operation of reading “00” is “1”, “1” is produced by thelogical operation using the lower order page data.

In short, the logic circuit for carrying out such logical operationsneeds to be so arranged that the value of the DS2 is stored in the datastorage section DS1 as lower order page data when DS3 is “1” and theoutcome of reading “01” is stored in the data storage section DS1 aslower order page data when DS3 is “0”.

FIG. 16 is a flow chart schematically illustrating the control algorithmof the first embodiment of flash memory according to the invention whenreading the higher order page data stored in a memory cell.

The control operation starts with receiving a read command from the hostand placing the read command in the state machine 8 (S1). Then, theoperation proceeds to receiving an address data from the host andplacing the address in the state machine 8 to select the page to be usedfor a read operation (S2). As a result of the addressing, the operationof Step S3 is automatically started by the state machine 8 in theinside.

Firstly, a “01” read operation is started in Step S3. The outcome of thereading operation shows upper order page data, which is stored in thecorresponding data storage section DS1. In other words, the outcome ofthe operation of reading “01” is used as upper order page data. Then,the data in the data storage section DS1 is externally output.

In this way, with the multi-value flash memory of the first embodiment,it is now possible to suppress any undesired increase of write time andreduce the distribution width of a threshold value so as to improve thereliability of the device.

Now, the second embodiment of the invention will be described below.

FIG. 17A is a graph illustrating the signal waveforms in a write step ofthe first embodiment of flash memory according to the invention asextracted from the signal waveform of FIG. 11. Note that the voltage ofthe bit lines BLe is made equal to 0.4V to carry out a second step writeoperation. In a write step of the first embodiment, the write operationis conducted while the voltage of the bit lines BL that is the writecontrol voltage is typically held to 0.4V during all the period ofapplying a predetermined write voltage (e.g., 18.0V as shown in FIG.17A) to the selected word line WL.

FIG. 17B is a graph illustrating the signal waveforms in a write step ofthe second embodiment of flash memory according to the invention. Asshown in FIG. 17B, the voltage of the bit lines BL that is the writecontrol voltage is held to 0V for only a predetermined period Twr out ofall the period of applying the write voltage Vpgm to the selected wordline WL and subsequently brought to Vdd in order to inhibit any writeoperation thereafter.

The predetermined period Twr for which the voltage of the bit lines BLis held to 0V is determined in such a way that the duration of thesecond step write operation is shorter that of the first step writeoperation. Then, the increment of the threshold value for the secondstep write operation can be made smaller than that of the threshold valefor the first-step write operation as in the case of the firstembodiment.

Thus, with the second embodiment, the effective value of the writecontrol voltage can be made substantially equal to that of the firstembodiment where the voltage of the bit lines BL that is the writecontrol voltage is held to a constant level during the entire write stepto consequently bring about the advantages of the first embodiment.

Now, the third embodiment of the invention will be described below.

FIG. 18 is a graph illustrating the signal waveforms of different partsof the third embodiment of flash memory according to the invention whenwriting a data into a single memory cell. It will be appreciated thatFIG. 18 corresponds to the waveforms of FIG. 11.

As described above by referring to FIG. 11, with the first embodiment,the voltage of the bit lines is reset to 0V after the end of a firststep write verify operation even when it maintains the voltage levelobserved immediately after a charging operation and then the bit linesare electrically recharged for a second step write verify operation.

On the other hand, with the third embodiment a write verify operation isconducted in a manner as described below.

The bit lines BLe are electrically charged typically to 0.7V for a firststep write verify operation. As the selected word line WL2 gets to thefirst step write verify voltage, the bit lines BLe maintain the 0.7V ifthe threshold value of the memory cell has got to the first step writeverify voltage. However, the voltage of the bit lines BLe falls toward0V if the threshold value of the memory cell has not got to the firststep write verify voltage. If the threshold value of the memory cell hasgot to the first step write verify voltage or not can be detected byobserving the voltage of the bit lines BLe by means of a sense amplifierat timing of tfv4 shown in FIG. 18. If the threshold value of the memorycell has got to the write verify voltage, the detecting operation issuccessfully completed.

Thereafter, at timing of tfv5 of tfv3, the voltage of the selected wordline WL2 is switched from the first step write verify voltage to thesecond step write verify voltage. For example, the voltage of theselected word line WL2 may be raised from 0.2V to 0.4V as shown in FIG.18. If the threshold value of the memory cell has got to the second stepwrite verify voltage, the 0.7V of the bit lines BLe is maintained. If,on the other hand, the threshold value of the memory cell has not got tothe second step write verify voltage, the voltage of the bit lines BLefalls toward 0V. If the threshold value of the memory cell has got tothe second step write verify voltage or not can be checked by detectingthe voltage of the bit lines BLe at the timing of tsv4. If the thresholdvalue of the memory cell has got to the write verify voltage, theoutcome of the detection is satisfactory.

The third embodiment provides an advantage of eliminating the timenecessary for charging the bit lines for a second step write verifyoperation and achieving a higher data writing rate in addition to theadvantages of the first embodiment. It will be appreciated that theabove description applies to a first or second step write verifyoperation with data “01” or data “00” by changing the write verifyvoltage.

While the above embodiments are described in terms of storing a 2-bitdata, or a 4-valued data, in a single memory cell, it will beappreciated that embodiments adapted to store a higher valued data in asingle memory can easily be realized.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A method for operating a non-volatile semiconductor memory devicecomprising a plurality of non-volatile semiconductor memory cells whichare connected in series, a plurality of word lines which are connectedto the plurality of non-volatile semiconductor memory cells, a firstselection gate which is connected between a bit line and one end of anarray of the plurality of serially connected non-volatile semiconductormemory cells, a first selection gate line which is connected to thefirst selection gate, a second selection gate which is connected betweena source line and another end of the array of the plurality ofnon-volatile semiconductor memory cells, and a second selection gateline which is connected to the second selection gate, comprising:writing data in a memory cell selected from the plurality ofnon-volatile semiconductor memory cells; performing a write verifyoperation which charges the bit line with a first voltage, applies asecond voltage to the first selection gate line and the second selectiongate line, and brings the first selection gate and the second selectiongate into conduction; sensing a voltage of the bit line aftermaintaining, for a predetermined time period, a state where a thirdvoltage is applied to the word line which is connected to the memorycell in which the data is written; and keeping the second voltageapplied to the first selection gate line and the second selection gateline, and sensing a voltage of the bit line after maintaining, for apredetermined time period, a state where a voltage of the word linewhich is connected to the memory cell in which the data is written ischanged from the third voltage to a fourth voltage.
 2. The methodaccording to claim 1, wherein the value of the fourth voltage is greaterthan that of the third voltage.
 3. The method according to claim 1,wherein the non-volatile semiconductor memory cells stores an n-valueddata, where n represents a positive integer not smaller than
 3. 4. Amethod for operating a non-volatile semiconductor memory devicecomprising a plurality of non-volatile semiconductor memory cells whichare connected in series, a plurality of word lines which are connectedto the plurality of non-volatile semiconductor memory cells, a firstselection gate which is connected between a bit line and one end of anarray of the plurality of serially connected non-volatile semiconductormemory cells, a first selection gate line which is connected to thefirst selection gate, a second selection gate which is connected betweena source line and another end of the array of the plurality ofnon-volatile semiconductor memory cells, and a second selection gateline which is connected to the second selection gate, comprising:writing data in a memory cell selected from the plurality ofnon-volatile semiconductor memory cells; performing a write verifyoperation which charges the bit line with a first voltage, applies asecond voltage to the first selection gate line and the second selectiongate line, and brings the first selection gate and the second selectiongate into conduction; sensing a voltage of the bit line aftermaintaining, for a predetermined time period, a state where a thirdvoltage is applied to the word line which is connected to the memorycell in which the data is written; and sensing a voltage of the bit lineafter maintaining, for a predetermined time period, a state where avoltage of the word line which is connected to the memory cell in whichthe data is written is changed from the third voltage to a fourthvoltage, without recharging the bit line with the first voltage.
 5. Themethod according to claim 4, wherein the value of the fourth voltage isgreater than that of the third voltage.
 6. The method according to claim4, wherein the non-volatile semiconductor memory cells stores ann-valued data, where n represents a positive integer not smaller than 3.7. A method for operating a non-volatile semiconductor memory devicecomprising a plurality of non-volatile semiconductor memory cells whichare connected in series, a plurality of word lines which are connectedto the plurality of non-volatile semiconductor memory cells, a firstselection gate which is connected between a bit line and one end of anarray of the plurality of serially connected non-volatile semiconductormemory cells, a first selection gate line which is connected to thefirst selection gate, a second selection gate which is connected betweena source line and another end of the array of the plurality ofnon-volatile semiconductor memory cells, and a second selection gateline which is connected to the second selection gate, comprising:writing data in a memory cell selected from the plurality ofnon-volatile semiconductor memory cells; performing a write verifyoperation which charges the bit line with a first voltage, applies asecond voltage to the first selection gate line and the second selectiongate line, and brings the first selection gate and the second selectiongate into conduction; sensing a voltage of the bit line aftermaintaining, for a predetermined time period, a state where a thirdvoltage is applied to the word line which is connected to the memorycell in which the data is written; and keeping the second voltageapplied to the first selection gate line and the second selection gateline, and sensing a voltage of the bit line after maintaining, for apredetermined time period, a state where a voltage of the word linewhich is connected to the memory cell in which the data is written ischanged from the third voltage to a fourth voltage, without rechargingthe bit line with the first voltage.
 8. The method according to claim 7,wherein the value of the fourth voltage is greater than that of thethird voltage.
 9. The method according to claim 7, wherein thenon-volatile semiconductor memory cells stores an n-valued data, where nrepresents a positive integer not smaller than
 3. 10. A method foroperating a non-volatile semiconductor memory device comprising aplurality of non-volatile semiconductor memory cells which are connectedin series, a plurality of word lines which are connected to theplurality of non-volatile semiconductor memory cells, a first selectiongate which is connected between a bit line and one end of an array ofthe plurality of serially connected non-volatile semiconductor memorycells, a first selection gate line which is connected to the firstselection gate, a second selection gate which is connected between asource line and another end of the array of the plurality ofnon-volatile semiconductor memory cells, and a second selection gateline which is connected to the second selection gate, comprising:charging the bit line with a first voltage, applies a second voltage tothe first selection gate line and the second selection gate line, andbrings the first selection gate and the second selection gate intoconduction; sensing a voltage of the bit line after maintaining, for apredetermined time period, a state where a third voltage is applied to aword line selected from the plurality of word lines; and keeping thesecond voltage applied to the first selection gate line and the secondselection gate line, and sensing a voltage of the bit line aftermaintaining, for a predetermined time period, a state where a voltage ofthe selected word line is changed from the third voltage to a fourthvoltage.
 11. The method according to claim 10, wherein the value of thefourth voltage is greater than that of the third voltage.
 12. The methodaccording to claim 10, wherein the non-volatile semiconductor memorycells stores an n-valued data, where n represents a positive integer notsmaller than
 3. 13. A method for operating a non-volatile semiconductormemory device comprising a plurality of non-volatile semiconductormemory cells which are connected in series, a plurality of word lineswhich are connected to the plurality of non-volatile semiconductormemory cells, a first selection gate which is connected between a bitline and one end of an array of the plurality of serially connectednon-volatile semiconductor memory cells, a first selection gate linewhich is connected to the first selection gate, a second selection gatewhich is connected between a source line and another end of the array ofthe plurality of non-volatile semiconductor memory cells, and a secondselection gate line which is connected to the second selection gate,comprising: charging the bit line with a first voltage, applies a secondvoltage to the first selection gate line and the second selection gateline, and brings the first selection gate and the second selection gateinto conduction; sensing a voltage of the bit line after maintaining,for a predetermined time period, a state where a third voltage isapplied to a word line selected from the plurality of word lines; andsensing a voltage of the bit line after maintaining, for a predeterminedtime period, a state where a voltage of the selected word line ischanged from the third voltage to a fourth voltage, without rechargingthe bit line with the first voltage.
 14. The method according to claim13, wherein the value of the fourth voltage is greater than that of thethird voltage.
 15. The method according to claim 13, wherein thenon-volatile semiconductor memory cells stores an n-valued data, where nrepresents a positive integer not smaller than
 3. 16. A method foroperating a non-volatile semiconductor memory device comprising aplurality of non-volatile semiconductor memory cells which are connectedin series, a plurality of word lines which are connected to theplurality of non-volatile semiconductor memory cells, a first selectiongate which is connected between a bit line and one end of an array ofthe plurality of serially connected non-volatile semiconductor memorycells, a first selection gate line which is connected to the firstselection gate, a second selection gate which is connected between asource line and another end of the array of the plurality ofnon-volatile semiconductor memory cells, and a second selection gateline which is connected to the second selection gate, comprising:charging the bit line with a first voltage, applies a second voltage tothe first selection gate line and the second selection gate line, andbrings the first selection gate and the second selection gate intoconduction; sensing a voltage of the bit line after maintaining, for apredetermined time period, a state where a third voltage is applied to aword line selected from the plurality of word lines; and keeping thesecond voltage applied to the first selection gate line and the secondselection gate line, and sensing a voltage of the bit line aftermaintaining, for a predetermined time period, a state where a voltage ofthe word line is changed from the third voltage to a fourth voltage,without recharging the bit line with the first voltage.
 17. The methodaccording to claim 16, wherein the value of the fourth voltage isgreater than that of the third voltage.
 18. The method according toclaim 16, wherein the non-volatile semiconductor memory cells stores ann-valued data, where n represents a positive integer not smaller than 3.